1. Field
The invention relates to highlight suppression circuits, and particularly to a circuit for controlling the beam current of a diode-gun camera tube and the like.
2. Prior Art
In camera pickup tubes typically exemplified by those tubes known as vidicons or plumbicons, the beam current of the electron beam which scans the target electrode thereof, is generally maintained within its normal operating range corresponding to the normal operating range of the video signal, in order to insure optimum tube resolution. That is, it is generally known that increasing the beam current of an electron beam which scans a target electrode above its given normal operating range generally causes a defocusing of the electron beam and thus a loss of resolution of the video picture.
In the event of the occurrence of highlights in the viewed scene, it is necessary that the photoelectric conversion surface of the target electrode be neutralized, generally by increasing the beam current to discharge the excess accumulation of charges on the target electrode of a pickup tube in the area of the highlight. However, one of the disadvantages inherent in pickup tubes when suppressing highlights by an increase in the beam current is that the use of a high intensity beam to discharge the excessive charges caused by the highlight also generates excessive soft x-rays, which bombard the target thereby significantly reducing the lifetime of the pickup tube. Thus, it is generally necessary to compromise the increase in beam current in order to provide the maximum highlight suppression possible while simultaneously maintaining the optimum lifetime of the pickup tube.
There are various techniques presently known which provide for highlight suppression in pickup tubes, wherein the techniques involve some means for increasing the beam current to discharge the excessive charges on the target electrode of the pickup tube caused by the presence of a highlight. The circuits corresponding to such techniques are commonly known by the general terms of anti-comet tail, automatic beam optimization, and automatic beam control, etc., circuits. Typical of the anti-comet tail circuits are those which increase the beam current during either the active or the flyback periods of the electron beam scan, and exemplified by way of example in U.S. patent application Ser. No. 802,839, to R. Salem, et al., and assigned to the same assignee as this application. This anti-comet tail circuit provides an increase in beam current only during the flyback period, and only "on demand", i.e., upon the occurrence of a highlight. Typical of automatic beam optimization and/or control circuits, are those described in U.S. Pat. Nos. 3,999,011 to K. Sato, et al., and 2,930,929 to C. Sheldon. Such circuits provide highlight suppression during the active period of the electron beam scan and in response to the presence of a highlight in the video picture being viewed. The circuits variously include some means by which the beam and/or cathode current is monitored, and the resulting control voltage signal is fed back to the grid of the tube via a generally complex negative feedback circuit which in turn provides the desired controlled increase in the beam as required to suppress the highlight effects.
All the above techniques are utilized with standard pickup tubes of the types known as vidicons, plumbicons, etc., which possess the characteristic that no current flows in the control grids between the grid voltage and the beam current. Whereupon the beam optimization circuits of previous mention utilize the generally linear relationship between the cathode current and the beam current to provide the negative feedback of control voltage to control the grid and thus the beam current.
Very recently, an improved camera pickup tube has been developed which is generally known as a diode-gun tube utilizing lead oxide target plumbicons, saticons, etc., wherein increased beam currents may be applied without unduly deteriorating the lifetime of the tube. Further, such diode-gun tubes have the characteristic that current flows in the control grid thereof, and a substantially linear relationship exists between the grid current and the available beam current.